1. Field of Invention
This invention relates generally to the mechanical and electrical structure of fluid drop ejectors.
2. Description of Related Art
Fluid ejection systems, such as inkjet printers, employ an array of electrically addressable ejectors that eject fluid onto a receiving medium, such as paper. In a thermal fluid ejection system, an electric current is applied to a resistive beater in the ejector head, vaporizing fluid in a fluid chamber. The rapid expansion of fluid vapor ejects a fluid drop through the fluid path and out the ejector opening. Alternatively, non-thermal fluid ejection systems rely on over-pressure due to mechanical compression caused by a piezoelectric element or thermo-mechanical pressure pulse to selectively eject a fluid drop from the ejector opening. Regardless of the apparatus for selectively ejecting fluid drop, both thermal and mechanical fluid ejectors share similar ejector geometries and ejected fluid characteristics.
In order to maximize throughput, fluid ejection systems eject fluid bi-directionally while traversing linear paths across the receiving medium. As a result, fluid is ejected during the full range of motion of the fluid ejection system.
Typically, in most fluid ejection systems, when a main drop is ejected, one or more smaller satellite drops are ejected at a deviated trajectory from that of the main drop. That is, the volume of ejected fluid breaks into a main drop and one or more smaller satellite drops. The deviation between the trajectories of the main and satellite drops generally remains constant for a given ejector geometry as the fluid ejection system moves. However, the perceived effect varies as the direction of motion of the fluid ejection system across the receiving medium changes. This produces a series of repeating alternating patterns aligned in the plane of motion of the fluid ejection system across the receiving medium. This phenomenon is known as banding. This effect is exacerbated when overall ejected fluid densities in a given swath are high, such as in image recording, as opposed to text recording, where overall ejected fluid densities are relatively low.
Various techniques have been proposed to eliminate the banding effect. In one technique, multiple passes are printed for each swath to average out the effect, so that each line contains both forward direction drop separation distances and reverse direction drop separation distances. However, this approach negatively impacts throughput and fluid consumption. In another technique, fluid is ejected only in that direction of motion of the fluid ejection system that minimizes the drop separation distance. Ejecting fluid in a single direction effectively eliminates the banding defect, but negatively impacts throughput. A third technique focuses on minimizing the forward direction and reverse direction drop separation distances by reducing the angle of separation as much as possible, and ideally to zero. This is accomplished by tightly controlling ejector head geometry, ejector head motion, fluid drop velocity, and other variables. However, this approach is susceptible to random variations in manufacturing tolerances and becomes more difficult as ejection speeds increase, as resolution increases or drop size is reduced.
The banding defect could be eliminated, even with a non-zero angle of separation, if the distance of separation between the main drop and satellite drop could be made to vary in the forward and reverse directions in such a way as to exactly compensate for the motion of the print head. However, with a fixed print head geometry, the angle of separation cannot be altered.
The inventors of this invention have determined that asymmetrical structures in the nozzle region tend to increase the angle of separation between the main drop and satellite drop.
Movable actuators in the ink path of an ink jet print head are known. They are typically used as flow control valves to selectively open and close the nozzle of the print head. U.S. Pat. Nos. 5,897,789 and 5,790,156 disclose such actuators. The 789 patent discloses minute active valve members operable to control ink flow within an inkjet printhead. In one embodiment, the valve assembly is incorporated in an ink channel that delivers ink to the firing chambers of the printhead. The 156 patent discloses an actuator-driven ink jet device that uses a piezoelectric material bonded to a thin film diaphragm. When a voltage is applied to the actuator, the actuator attempts to change its planar dimensions, causing the actuator to deform about its fixed end. This displaces ink in the chamber, causing ink to flow both through an inlet from the ink supply to the ink chamber and through an outlet and passageway to a nozzle.
This invention provides systems and methods that vary an internal geometry of the fluid path of a fluid ejector.
This invention separately provides systems and methods that vary relative ejection trajectories of satellite and main drops ejected by a fluid ejection system.
This invention further provides systems and methods that vary the relative trajectories to reduce differences in drop separation distances between the satellite and main drops.
This invention further provides systems and methods that vary the relative ejection trajectories to obtain substantially constant drop separation distances between the satellite and main drops.
This invention further provides systems and methods that vary the relative ejection trajectories of the main and satellite drops based on a direction of motion of an ejector head that ejects the fluid drops.
This invention further provides systems and methods that vary the internal geometry of an ejector system to control differences in drop separation distances between the main and satellite drops.
This invention further provides systems and methods that vary the internal geometry of an ejector system to obtain a substantially constant drop separation distances between the main and satellite drops.
This invention further provides systems and methods that vary the internal geometry of an ejector system based on a direction of motion of an ejector head that ejects the fluid drops.
This invention further provides systems and methods that vary the internal geometry of an ejector system based on a direction of motion and a velocity of an ejector head that ejects the fluid drops.
This invention separately provides systems and methods that vary the internal geometry of an ejector system to controllably vary the relative trajectories of the main and satellite drops.
This invention separately provides systems and methods that vary the internal geometry of an ejector system by controllably actuating a mechanical actuator located within the fluid path.
This invention further provides systems and methods that vary the internal geometry of an ejector system by controllably energizing a bimetallic element.
This invention further provides systems and methods that vary the internal geometry of an ejector system by controllably energizing a piezoelectric elements or a micro-electromechanical system.
In various exemplary embodiments, a controllable actuator is placed into the fluid path of each fluid ejector in a fluid ejector head. The controllable actuator is controllably actuated or energized to cause the actuator to alter the internal geometry of the fluid path. In various exemplary embodiments, the degree to which the internal geometry is altered is controllable based on the degree to which the actuator is actuated or energized.
By altering the internal geometry of the fluid path, the angle of separation, and thus the drop separation distance, changes. In various exemplary embodiments, the actuators are operated to reduce, and ideally hold constant, the drop-separation distance as the fluid ejector head moves in forward and reverse directions across the receiving medium. In various exemplary embodiments, in a direction of motion that tends to increase the drop separation distances, the actuators are operated to minimize the angle of separation. In contrast, in a direction of motion that tends to reduce the drop separation distance, the actuator is operated to increase the angle of separation such that the drop separation distance in that direction becomes closer to the drop separation distance in the other direction, and ideally is the same.
In various exemplary embodiments, the actuators are formed using bimetallic structures. When such bimetallic actuators are not actuated or energized, they assume a rest position, which, in various exemplary embodiments, is along a surface of the fluid path. When energized, such bimetallic elements bend away from the rest position due to differing coefficients of thermal expansion. In various exemplary embodiments, the energized bimetallic elements bend into the fluid path to alter the flow of the fluid as it is ejected from the ejector.
In various other exemplary embodiments, the actuators are formed by piezoelectric elements or micro-electromechanical systems (MEMS). When such piezoelectric or MEMS are not energized, these devices assume a rest position. In various exemplary embodiments, the rest position is substantially outside of the flow of fluid through the ejector. When energized, such piezoelectric elements or MEMS deform to extend into the fluid passage, to alter the fluid flow of the fluid as it is ejected from the ejector.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.